The present invention relates to an improved method for removal of nitrates (denitrification) from water that contains nitrates as pollutants.
The nitrate content in underground and surface water is continuously increasing, mainly due to the synthetic or natural fertilizers used in intensive-cultivation agricultural areas.
Ingestion of nitrates by children (infants and young children) is a severe health hazard due to the reduction of nitrates to nitrites and to the subsequent oxidation of hemoglobin to metahemoglobin, which is a stable form and is no longer able to transfer oxygen to the tissues (metahemoglobinemia).
In adults, metahemoglobin is rapidly converted to the oxidized form, which carries oxygen in the blood; accordingly, the World Health Organization believes that a moderate content of nitrites in potable water is not a health risk for adults.
The maximum limit set for nitrites by European Community standards is 50 mg/l; the recommended limit is 25 mg/l and 10 ml/l for children.
The most widely used methods for removing nitrates from water are biological or are based on ion exchange or on the use of reverse-osmosis membranes.
In the case of ion exchange and reverse osmosis, the problem of removing nitrates is transferred elsewhere, since the nitrate-rich water that these processes generate must in turn be disposed.
Biological denitrification uses bacteria which, in an anoxic environment, use organic carbon for growth (heterotrophic processes) or inorganic carbon (autotrophic processes).
In heterotrophic processes, the oxygen that is present in the nitrate molecule is used to oxidize the organic substance (generally sugar) and the nitrates are gradually reduced to elementary nitrogen.
The considerable production of biomass and sludge to be disposed and the frequent countercurrent washing in order to minimize the formation of nitrites are the main limitations of heterotrophic processes.
Moreover, these processes, like autotrophic ones, are characterized by very slow rates.
In autotrophic processes in which the source of inorganic carbon is generally constituted by dissolved carbon dioxide or by the bicarbonates themselves that are present in the water, hydrogen is mostly used as reducing agent.
Only a few microorganisms are capable of surviving and growing in a hydrogen environment: this determines a biological selection of the usable bacteria. Schematically, in autotrophic processes the water is pumped into a pressurized tank, where it is saturated with hydrogen; before entering the reactor, the water receives the addition of carbon dioxide in order to compensate for the loss of H30  ions caused by the denitrification reactions.
The bacterial biomass adheres to a filtration bed with high surface area.
The denitrified water then enters an aeration tank, which reoxygenates the water and strips away the residual oxygen; it then passes over gravity filters which retain any biomass and pollutants that can be filtered out. The last step of the process consists of disinfection with ultraviolet rays in order to inactivate any residual microorganisms.
The autotrophic process does not produce large amounts of sludge to be disposed with respect to the heterotrophic one.
Moreover, the risk of bacterial contamination is reduced, since the bacteria capable of surviving in the presence of hydrogen are few and generally consist of the same strains that are present in limited concentration in potable water.
The slowness of the process (generally, 1-2 kg of nitrates removed per m3 of supported biomass per day) is one of its main limitations.
In order to obviate the drawbacks of biological processes, a catalytic process has been proposed in which the nitrates are reduced with hydrogen, using catalysts based on palladium and copper on carriers having a controlled (bimodal) pore radius distribution or on activated carbon (U.S. Pat. No. 4,990,266).
The above indicated catalysts, however, tend to generate ammonia in an amount which exceeds the limits set by the applicable statutory provisions (0.5 mg/l): this occurs when working with amounts of hydrogen exceeding certain limits.
In order to obviate the above mentioned difficulties, catalysts are used which comprise palladium (which is capable of catalyzing the reduction of the nitrites, but not of the nitrates, to elementary nitrogen with high selectivity) mixed with catalyst which comprise palladium and copper, which are capable of catalyzing nitrate reduction (S. Horold et al, Catalysis Today, 17 (1993), 21-30).
The generation of ammonia in amounts exceeding the allowed limits, which occurs if the hydrogen concentration exceeds even low limits at which good nitrate removal activity occurs, causes the process to be of limited practical interest.
The aim of the present invention is to provide a water denitrification process which can reduce, with high specific activities, nitrates to elementary nitrogen, avoiding the generation of ammonia beyond the limits allowed by the applicable statutory provisions and/or of toxic substances of any other type.
An object is to provide a method as indicated above which can be performed continuously and at a convenient cost.
This aim, this object and others are unexpectedly achieved with the denitrification process according to the invention, which comprises a stage in which the water containing the nitrate ions to be eliminated is made to flow over an inert porous carrier which supports a transitioon metal capable of activiting the molecular hydrogen, forming metallic hydrides, and on which denitrifying bacterial strains capable of surviving in anoxic conditions in the presence of hydrogen are made to adhere and grow.
The process is performed in the presence of hydrogen as a reducing agent, used in the quantity stoichiometrically required by the denitrification reaction or in a larger quantity, adjusting the pH of the reaction to values between 4.5 and 7.8, preferably with carbon dioxide.
The global denitrification reaction occurs according to the following equation: 2NO330 5H2xe2x86x92N22OH+4H2O.
The amount of hydrogen that is stoichiometrically required to reduce to nitrogen 100 mg of NO3 is 8.1 mg.
The hydrogen is preferably used in excess quantities with respect to the stoichiometric quantity in order to ensure complete reduction of the nitrates to elementary nitrogen.
The bacterial strains are used in quantities which are sufficient to reduce the nitrates to elementary nitrogen, working in the presence of hydrogen, in the absence of transition metal.
Examples of usable bacteria are Pseudomonas fluorescens, Xanthomonas maltophila, Flavobacterium indologenes, Alcaligens eutrophus, Pseudomonas maltophila and Pseudomonas putrefaciens. 
Usable carriers are constituted by inert materials with high porosity (higher than 0.3 cm3/g) and a surface area of more than 30 m2/g.
The carriers can be inorganic (porous oxides), such as silica and alumina, or polymeric, such as partially cross-linked styrene-divinyl benzene resins (which can be prepared for example according to the method described in U.S. Pat. No. 4,224,415) or expanded polypropylene with high surface area such as the Accurel((copyright)) product by Akzo, which has an area of 90 m2/g, a porosity of 0.7-0.8 cm3/g and a cell size of 1-10 microns.
Carriers such as alumina and silica preferably have bimodal porosity with over 20% of their pores having a radius of more than 2000 A.
Carriers capable of adsorbing nitrates at least partially are preferred.
Activated carbon is the preferred carrier, due to its good ability to adsorb both nitrates and hydrogen.
Hydrogen adsorption allows to feed the hydrogen directly into the denitrification reactor, obtaining uniform distribution in the water to be denitrified without having to first solubilize the hydrogen in the water to be denitrified with methods capable of providing injection in the form of very fine bubbles.
The preferred activated carbon is of vegetable origin (coconut carbon) with a porosity of 0.6 to 0.8 cm3/g and a surface area of 600 to 1100 m2/g.
The carbon is generally in the form of flakes measuring a few millimeters.
The preferred metal is palladium, which is used in an amount between 0.001 and 5% by weight on the carrier, preferably 0.05-0.5%.
It should be noted that the palladium, in the absence of the denitrifying bacteria, is not capable of catalyzing the reduction of the nitrates to elementary nitrogen; it catalyzes nitrite reduction instead.
Accordingly, the synergistic effect of palladium on the denitrifying activity of the bacteria is surprising.
Other usable metals comprise ruthenium, osmium and platinum.
The quantity of these metals is between 0.01 and 5% by weight, preferably 0.1 to 1%.
Palladium can be used in a mixture with other metals, such as Cu and Ag, used in smaller molar proportions than the palladium.
The metals are supported by means of known methods, for example by impregnating the carrier with the dry impregnation method, i.e., by using the solution of the metallic salt in a quantity equal to, or lower than, the volume of the pores.
Reduction to metal is also provided according to known methods, generally using hydrogen as reducing agent.
The growth of bacterial strains on the catalytically active carrier is also performed according to known methods, for example by using nutrients based on sodium phosphate, glucose and/or methanol which can be introduced, with appropriate dosages, so as to ensure a sufficient concentration in input to the denitrification reactor.
The pH is kept within the range of 4.5 to 7.8, preferably with CO2 or with acids which supply anions which are commonly present in potable water, such as for example hydrochloric acid.
The use of CO2 is preferred, since CO2 also acts as a nutrient for bacterial proliferation.
The amount of CO2 to be fed is such as to ensure that the pH remains at the preset value.
A schematic flowsheet of a plant operating according to the process of the invention comprises a reactor which contains the catalytically active carrier on which the bacterial biomass adheres, in which the water enters from below and flows through the catalytic mass with an upward flow in equicurrent with the flow of hydrogen and carbon dioxide.
The amount of CO2 fed is such that the reactor is under a pressure of CO2 (1-5 bar).
The water at the output of the reactor is subjected to flashing in order to recover the carbon dioxide and unreacted hydrogen, which are recycled.
The nitrogen generated during the reaction is partly vented, since an excessive concentration in the solution does not facilitate the denitrification action, since nitrogen is a product of said reaction.
The efficiency of the reduction of the nitrates to elementary nitrogen is preferably limited to 80-90%.
The nitrites that form as an intermediate product are eliminated in a subsequent step, preferably by oxidation with ozone.
It is also possible to use other known denitrification methods.
The specific activity of the process is greater than 5 kg of NO3xe2x88x92removed per m3 of reactor per day.
The process can be used not only for the denitrification of water for drinking; it can also be used for industrial water for beverages or for foods or for wastewater downstream of the chemical-biological treatment to which said wastewater is subjected to oxidation to nitrate of ammonia and similar nitrogen impurities contained in said wastewater.
The following examples are provided as non-limitative illustration of the invention.